1. Field of the Invention
This invention relates to non-impact printers which are used for recording on photosensitive recording surfaces by electro-optical means such as light-emitting diodes and other light sources. The invention further relates to a method for making such printers to provide balanced light outputs by the recording elements.
2. Description Relative to the Prior Art
In the prior art as exemplified by DE 34 21 914 A1, an LED (light-emitting diode) printhead is disclosed wherein the problem of nonuniformity of light output from the LED's is recognized. Each printhead may include several thousand LED's arranged in a row. When the LED's are driven with the same levels of electrical current, some of the LED's are found to provide more light output than others, thereby providing nonuniformity between their respective exposures on the recording surface. To overcome this problem, the patent publication discloses that a photographic mask may be provided between the LED's and the recording surface. The mask comprises a photographic film that has been exposed by the LED's. The density of the portion of a mask opposite a resPective LED varies with the brightness of that LED so that correspondingly more light is absorbed in masked portions opposite brighter LED's. Thus, uniformity in light output is achieved or at least improved.
The problem with the use of photographic film in this type of apparatus is that the film is required to be separately developed and then must be accurately repositioned.
To overcome this problem, it has been suggested in U.S. Pat. No. 4,378,567 to provide recording elements coated with phtobleachable dyes. The dyes are photobeleached and fixed at density levels compensating for the nonuniformities between the recording elements. A problem with this apparatus, as applied at least to LED's, is that LED's over time will age differently and the balance in uniformity achieved initially will subsequently be lost.
It is an object, therefore, of the invention to provide for improved uniformity in LED printheads or the like without the noted disadvantages of the prior art.